As a method for reducing intermodulation distortions occurring in an amplifier, there is known a method where a distortion compensation circuit, which generates intermodulation distortions having opposite characteristics to intermodulation distortions occurring in the amplifier, is connected to a stage previous or subsequent to the amplifier.
The amplitude of the intermodulation distortions of the opposite characteristics generated by the distortion compensation circuit is the same as the amplitude of the intermodulation distortions occurring in the amplifier. The phase of the intermodulation distortions of the opposite characteristics is different by 180 degrees from the phase of the intermodulation distortions occurring in the amplifier.
FIG. 19 is a configuration diagram showing a distortion compensation circuit disclosed in Patent Literature 1 mentioned later. FIG. 20 is a configuration diagram showing a distortion generator circuit in the distortion compensation circuit of FIG. 19.
In the distortion compensation circuit, a distortion generator circuit 103 that generates intermodulation distortions having opposite characteristics to intermodulation distortions occurring in an amplifier is inserted between an input terminal 101 and an output terminal 102.
This distortion compensation circuit includes an impedance conversion circuit 104 that changes the impedance for a frequency band corresponding to a difference frequency (f2−f1) of two-wave RF signals (i.e., an RF signal with a frequency f1 and an RF signal with a frequency f2), which are input through the input terminal 101.
Since the impedance conversion circuit 104 is provided, the amplitude and phase characteristics of the intermodulation distortions generated by the distortion generator circuit 103 can be adjusted without affecting the characteristics of frequency bands used by the two-wave RF signals.
Therefore, by adjusting the amplitude and phase characteristics of the intermodulation distortions generated by the distortion generator circuit 103 as appropriate in dependence on the intermodulation distortions occurring in the amplifier, the intermodulation distortions occurring in the amplifier can be accurately compensated for.
The impedance conversion circuit 104 is composed of an inductor 105, a resistor 106, and a capacitor 107.
The inductor 105 and the capacitor 107 have individual unique self-resonant frequencies, and do not function as an inductor and a capacitor unless the frequencies f1 and f2 of RF signals are equal to or lower than the self-resonant frequencies. In general, the self-resonant frequencies tend to decrease as the values of the inductor and capacitor increase due to the influence of parasitic capacitance, etc.
The frequency band corresponding to the difference frequency (f2−f1) of two-wave RF signals (an RF signal with the frequency f1 and an RF signal with the frequency f2) input through the input terminal 101 is several hundred MHz or less.
Hence, to change impedance in that frequency band, there is a need to use the inductor 105 and the capacitor 107 that have large values. However, since the frequencies f1 and f2 of the two-wave RF signals are normally several GHz or more, the frequencies f1 and f2 are expected to exceed the unique self-resonant frequencies of the inductor 105 and the capacitor 107.